WATER JUNE 2014
68
Technical Papers
Collection System #1. Although the
gure is very busy, with a large amount
of information graphically and spatially
displayed, the gure allowed for the
system to be viewed as a whole. This
information was then used to de ne
potentially better mitigation 'solutions'
for Collection System #1.
Corrosion and odour management
solutions considered
Multiple types of solutions are technically
feasible. To consider all of them and
calculate their cost ef ciency would take
an enormous amount of resources. For
this reason the Hierarchy of Controls
was developed (see Figure 3). Each
technology has its particular niche,
and all are described in the Guideline.
Essentially, the solutions that are
considered are:
• Source Control -- limiting the discharge
of industrial and commercial customers
to < 600 mg/L BOD, pH >7.0 and
temperature < 30°C (trade waste
control measures);
• Limit Hydraulic Retention Time (HRT)
-- sewer design and operation;
• Limit sul de generation -- biocide
dosing, high scour velocity and
ushing;
• Limit sul de transfer into the gas phase
-- reduced turbulence, pH adjustment
dosing (eg. Mg(OH)2), precipitation
dosing (eg. iron salt), oxidative dosing
(eg. Ca(NO3)2 or O2);
• Corrosion-resistant coatings
and/or materials;
• Sacri cial coatings (eg, Mg(OH)2);
• Forced ventilation with foul air
treatment such as:
-- Activated carbon ltration
-- Biotricking ltration
-- Wet chemical scrubbing.
During the process to select the best
solution, non-cost considerations are
also taken into account. For example,
the ability for the solution to deal with
operational changes and operational
upsets. Iterative model runs are used
to quantify the sensitivity and robustness
for these potential risks.
Base case and alternative management
strategies
First, the current network management
practice was modelled and calibrated
against eld data. Field data at speci c
key locations in the network was
obtained in summer and winter. The
eld data, together with historical data,
was analysed for different parameters,
among them sul de concentration,
water ow, temperature and H2S in
the gas phase. The current network
management practice involves chemical
dosing at two locations far upstream
(with one main ferrous chloride dosing
point far upstream in the main trunk),
and extraction of foul air downstream
at the wastewater treatment plant.
After the current network management
practice was modelled, mapped and
costed over a 30-year period, it was
compared to alternative management
strategies. Also, the "do nothing"
scenario, in which existing measures
(in this case chemical dosing and foul air
extraction) are eliminated, was evaluated.
Figure 7 shows the maximum H2S
concentrations (95 percentile) that can be
expected in the headspace of the pipes
in the network for this scenario. Figure
8a shows the outcome map in which
the increase in projected rehabilitation
costs compared to current network
management practices is illustrated.
Each coloured square in the output
map represents 1 square km. When the
colour is red, the rehabilitation costs
will be higher than the current network
management practice. When the color
is green, the rehabilitation costs will
be lower than the current network
management practice. The darker the
Figure 6. The output of the rst step (problem identi cation)
for Collection System #1.
Figure 7. The maximum H2S concentrations (90%ile) that can be expected in the
headspace of the pipes (>350mm) in the network for the scenario "do nothing".
ODOUR MANAGEMENT